1. Field of the Invention
The present invention relates generally to a communication system using orthogonal frequency division multiplexing (OFDM) scheme, and more particularly to an apparatus and a method for generating a pilot signal (or pilot pattern) for distinguishing a base station.
2. Description of the Related Art
An OFDM scheme has recently been used as a data transmission scheme using multi-carriers for high-speed data transmission in a wired and/or wireless channel. More specifically, the OFDM is a multi-carrier modulation scheme in which symbols input in series are serial-to-parallel converted in order to be modulated into a plurality of sub-carriers being orthogonal to each other, i.e., a plurality of sub channel, which are then transmitted.
The OFDM scheme is widely used in digital transmission technologies, such as digital audio broadcasting (DAB), digital televisions, wireless local area network (WLAN), and wireless asynchronous transfer mode (WATM). That is, conventionally, the OFDM scheme was not widely used due to the complexity of the required hardware, but it has been recently realized because of the development of various digital signal processing technologies including a fast Fourier transform (FFT) and an inverse fast Fourier transform (IFFT).
The OFDM scheme is similar to a conventional frequency division multiplexing (FDM) scheme but can achieve an optimum transmission efficiency because a plurality of sub-carriers are transmitted while maintaining orthogonality between them in the OFDM. That is, the OFDM scheme is efficient in its use of frequencies and is highly resistant to multi-path fading, thereby achieving optimum transmission efficiency in high-speed data transmission.
Further, because the OFDM scheme uses an overlapped frequency spectrum, it is efficient in its use of frequencies and is highly resistant to frequency selective fading and multi-path fading. In addition, the OFDM scheme can reduce inter symbol interference (ISI) influence by utilizing a guard interval and enables the hardware structure of an equalizer to be simply designed. Furthermore, because the OFDM scheme is highly resistant to impulse noise, it has been actively utilized in the structure of a communication system.
Hereinafter, operations of a transmitter and a receiver of a communication system using the OFDM scheme, i.e., an OFDM communication system, will be briefly described.
In the transmitter of the OFDM communication system, input data is modulated into sub-carriers via a scrambler, an encoder, and an interleaver. The transmitter provides various variable data rates and has different code rates, interleaving sizes, and modulation schemes according to the data rates.
Conventionally, the encoder uses a code rate of ½, ¾ etc., and the size of the interleaver for preventing burst error is determined depending on the number of coded bits per symbol (NCBPS). The modulation scheme uses a quadrature phase shift-keying (QPSK) scheme, an 8 phase shift keying (8PSK) scheme, a 16 quadrature amplitude modulation (16QAM) scheme, a 64 QAM scheme, etc.
A predetermined number of pilot sub-carriers are added to a signal modulated into a predetermined number of the sub-carriers by the aforementioned elements. Then, the signal including the pilot sub-carriers passes through an IFFT block to generate an OFDM symbol. A guard interval for eliminating ISI in multi-path channel environments is inserted into the OFDM symbol. The OFDM symbol passes through a symbol waveform generator and is finally input to a radio frequency (RF) processor. The RF processor RF-processes an input signal and sends the processed signal through the air.
The receiver of the OFDM communication system corresponding to the transmitter as described above performs a process inverse to the process performed by the transmitter and has an additional synchronization process. First, the receiver must perform a process in which frequency offset and symbol offset are estimated for the received OFDM symbol by means of a preset training symbol. Then, data symbols from which the guard interval has been eliminated pass through an FFT block and are restored to a predetermined number of sub-carriers including a predetermined number of pilot sub-carriers.
Further, in order to overcome path delay on a radio channel, an equalizer estimates a channel state for a received channel signal and eliminates signal distortion on the radio channel from the received channel signal. Data for which the channel estimation has been performed by the equalizer is converted into a bit sequence, passes through a de-interleaver, and is output as final data via a decoder and a de-scrambler for error correction.
As described above, in an OFDM communication system, a transmitter (i.e., base station) transmits pilot sub-carrier signals (pilot channel) to a receiver (i.e., a terminal). That is, the base station transmits data sub-carrier signals (data channel) and simultaneously transmits the pilot sub-carrier signals. Herein, the pilot sub-carrier signals are transmitted for synchronization acquisition, channel estimation, and base station differentiation.
Further, the pilot signals operate as a kind of training sequence and enable channel estimation to be performed between the transmitter and the receiver. The terminal may differentiate a base station to which the terminal itself belongs using the pilot signals. A position at which the pilot signals are transmitted has been predetermined between the transmitter and the receiver. As a result, the pilot signals operate as a kind of reference signal.
The base station transmits the pilot signals with a relatively high transmission power than that for data signals, which enables the pilot signals to reach even a cell boundary, while the base station enables the pilot signals to have a specific pattern, that is, a pilot pattern.
The high power transmission of the pilot signals, even with a specific pilot pattern, enables the pilot signals to reach the cell boundary. That is, when the terminal enters a cell, the terminal does not have any information on a base station to which the terminal itself currently belongs. Accordingly, in order to detect a base station at which the terminal is currently located, the terminal must use the pilot signals. Therefore, the base station transmits the pilot signals with relatively high transmission power in order to have a specific pilot pattern, such that the terminal can detect the base station to which the terminal itself belongs.
Further, the pilot pattern is by the pilot signals transmitted from the base station. That is, the pilot pattern is differentiated by a slope of the pilot signals and a starting point at which the pilot signals are transmitted. Accordingly, in the OFDM communication system, in order to enable base stations included in the OFDM communication system to be differentiated from each other, the base stations must be designed to have pilot patterns different from each other.
Further, the pilot pattern is generated based on a coherence bandwidth and a coherence time. Hereinafter, the coherence bandwidth and the coherence time will be described.
The coherence bandwidth represents a maximum bandwidth in which the identity of channels can be assumed on a frequency domain, that is, the invariability of channels can be assumed. The coherence time represents a maximum time for which the identity of channels can be assumed on a time domain, that is, the invariability of channels can be assumed.
As described above, because the identity of channels can be assumed in the coherence bandwidth and the coherence time, even though only a pilot signal is transmitted in the coherence bandwidth and the coherence time, no problem occurs in synchronization acquisition, channel estimation, base station differentiation, etc. Further, because data signals can be maximally transmitted, the entire performance of a system can be improved.
As a result, a minimum frequency separation enabling the transmission of pilot signals is a coherence bandwidth and a minimum time separation (i.e., minimum OFDM symbol time separation), enabling the transmission of the pilot signals is a coherence time.
The number of base stations included in the OFDM communication system is changed according to the size of the OFDM communication system. That is, the number of the base stations increases as the size of the OFDM communication system increases. Accordingly, in order to differentiate the base stations from each other, pilot patterns having different slopes and starting points must exist by the number of the base stations.
However, in order to transmit pilot signals on a time-frequency domain in the OFDM communication system, the coherence bandwidth and the coherence time must be considered as described above. When the coherence bandwidth and the coherence time are considered, the number of the pilot patterns having different slopes and starting points is limited.
When pilot patterns are generated without considering the coherence bandwidth and the coherence time, pilot signals are scattered in pilot patterns representing different base stations. In such a case, it is impossible to distinguish base stations from each other by means of the pilot patterns.
FIG. 1 is a view illustrating possible slopes of pilot patterns in a conventional OFDM communication system. Referring to FIG. 1, the possible slopes of pilot patterns and the number of the slopes, that is, the slopes and the number of the slopes according to the transmission of a pilot signal, depend on a coherence bandwidth and a coherence time. In a case in which the coherence bandwidth is six and a coherence time slot is 1, when it is assumed that the slope of a pilot pattern has a value of integer, the number of slopes of a pilot pattern capable of being generated under the condition is six from s=0 to s=5. That is, the slope of the pilot pattern capable of being generated under the condition has one integer of 0 to 5. As described above, because the number of slopes of a pilot pattern capable of being generated under the condition is six, the number of base stations capable of being differentiated by means of the pilot pattern is six in the OFDM communication system satisfying the condition.
Hereinafter, a pilot sub-carrier in which the slope of a pilot pattern is six will be described. Actually, because there is no difference between a case (s=0) in which the slope of a pilot pattern is zero and a case in which the slope (s=6) of a pilot pattern is six, only one slope of the two cases can be used. That is, as described above, the pilot sub-carrier in which the slope of the pilot pattern is six is equal to another pilot sub-carrier in which the slope of a pilot pattern spaced the coherence bandwidth apart from the pilot sub-carrier is zero. Therefore, the case where s=0 and the case where s=6 cannot be distinguished from each other.
The circle hatched by oblique lines illustrated in FIG. 1 represents pilot sub-carrier signals spaced by the coherence bandwidth. That is, a case in which the slope of a pilot sub-carrier marked by the white circles is six is equal to a case in which the slope of the pilot sub-carrier marked by the circle having the oblique is zero. Accordingly, the slope of the pilot sub-carrier is limited to the coherence bandwidth.
Herein, all slopes enabling the generation of the pilot pattern can be expressed by Equation (1).
                              S          val                =                  [                      0            ,            …            ⁢                                                  ,                                                            B                  c                                                  T                  c                                            -              1                                ]                                    (        1        )            
In Equation (1), Sval represents the slope of a pilot pattern capable of being generated in the OFDM communication system. Herein, it is preferred that the slope of the pilot pattern has a value of integer, but it is not always necessary that the slope of the pilot pattern has a value of an integer. Further, in Equation (1), Tc represents a coherence time, that is, the number of basic data units constituting the coherence time on a time domain.
In FIG. 1, a basic data unit included in the coherence time is an OFDM symbol and Tc represents the number of OFDM symbols. Further, in Equation (1), Bc represents a coherence bandwidth, that is, the number of basic sub-carrier units constituting the coherence bandwidth on a frequency domain.
The actual maximum number of slopes enabling the generation of a pilot pattern can be expressed by Equation (2).
                              S          no_max                =                              B            c                                T            c                                              (        2        )            
In Equation (2), Sno—max represents the maximum number of slopes enabling the generation of the pilot pattern in the OFDM communication system.
Therefore, as described above, because the pilot patterns used to differentiate base stations included in the OFDM communication system in the OFDM communication system are generated depending on the coherence bandwidth and the coherence time, there is a limitation in the number of generable pilot patterns. Accordingly, when the number of the base stations included in the OFDM communication system increases, the number of distinguishable base stations is limited due to the limitation in the number of the generable pilot patterns. Further, because adjacent base stations may have the same pilot patterns, it is impossible to distinguish the base stations from each other. Moreover, it is difficult to exactly estimate a channel due to interference between adjacent cells.